In the art of joining metals, brazing and welding are certainly common and well known methods. Brazing differs from welding in that brazing does not involve melting the metals being joined and welding does. The metal elements being joined are known as the base metal. When rods are joined end to end, the joint between them is known as a butt joint. Typically, when rods are joined end to end, they are either welded or they are brazed. Welding metal rods together involves melting and re-solidifying the rod base metal itself; some metal of each of the rods is melted and solidified such that a new solid joint is formed between the two rods when the molten metal solidifies. The weld which produces such an end to end joint is know as a butt weld.
There are times, however, when welding is not the most desirable method for forming the joint so described. Melting the parent metal can produce undesirable characteristics in the joint so formed. In such cases, the alternative to welding is brazing.
Brazing is a process which joins two metals together by creating a metallurgical bond between an intermediate filler material and the two metals being joined. In brazing, the parent metal is not melted. Since the parent metal of the material being joined is not melted, the joining process takes place at lower temperatures than in the case of welding the exact same material. In a simple form, the mechanism of forming a butt joint by brazing comprises two masses of metal, separated by a narrow, but defined, substantially uniform gap. The two masses are heated to a temperature high enough to melt a brazing filler material. When the masses are sufficiently hot, the filler material is brought into contact with the hot masses whereon the filler material is melted. The spacing between the two masses is configured such that when the brazing filler material melts, it (the filler material) is drawn into the gap between the two masses by capillary action. Upon solidification of the brazing filler, a metallurgical bond is established between the filler material and each metal mass. Since each metal mass bonds to the filler material, and the filler material is positioned between the two masses, it follows that the bond which is created between each mass and the filler results in each mass being bonded, by way of the filler, to each other.
The quality of the brazed joint depends upon many factors. Some factors are: the uniformity of the two faces being joined; the alignment of the faces being joined; the spacing of the two faces being joined; and the propensity of the metal being joined to form a metallurgical bond with the brazing filler material being used.
The propensity of the metal being joined to form a metallurgical bond with the brazing filler material being used, as well as the heating techniques, will be a function of selecting the proper filler for the application at hand and using the proper heating techniques. It will be presumed that the filler and its propensity to bond with the parent metal will be suitable, as will the suitability of the heat and temperature of the process.
The quality of the resulting joint is thereafter dependent upon the preparation and execution of proper techniques in preparing the base metal for the brazing process. The uniformity of the bonding faces of the metal being joined and their position during the brazing operation cannot be overemphasized. In joining solid rods, the ends being joined must be prepared and cut such that there are no rounded corners. The cut face should have neither high spots nor low spots which would result in a significant variation of the distance from one face being joined to the other. Given a constant and acceptable chemistry of the metals involved in forming the joint, the degree of the metallurgical bond is ultimately dependant upon the flatness of the surface of the face of the rods being joined and how well they are positioned during the brazing process. If the two faces being joined are not sufficiently flat, there will be thick and thin areas of the brazing filler material as well as the possibility of areas having no filler at all. If the two faces are not sufficiently equidistantly positioned, there will be variation in the thickness of the gap therebetween and drawing the filler into the gap by capillary action may be compromised. With uneven faces, the bond obtained may also be compromised.
It should also be obvious that alignment of the faces is another critical element of the joint. If the faces are aligned such that the spacing on one side of the joint is too wide, there may be no uniform capillary action to draw the filler into the entirety of the joint. No filler in parts of the joint means no metallurgical bond at those parts. No bond in part of the joint necessarily produces a weaker joint than one wherein the entirety of the faces are bonded. If the alignment is such that no capillary action results when the molten filler is exposed thereto, the result would again be no filler at all drawn into the joint. Again, no filler means no bond and no joint.
The gap spacing between the faces is another factor affecting the quality of the joint. Even with proper facial and axial alignment of rods and rod faces being joined, lack of control of the dimensions or uniformity of the gap between the two faces will certainly adversely affect the formation of consistent, high quality joints.
Manufacturing requirements dictate that production processes be simple and free of as many opportunities to introduce errors as possible. In joining solid rods end to end, the cut faces of the rods can be virtually any angle as measured from the centerline of the rod itself. The only real restriction is that both rods must be cut at substantially the same angle if the resulting length of rod is to be substantially straight. Any angle chosen will be a compromise of a plurality of considerations. If the chosen angle is 90 degrees, that is to say that each section of the rod is cut substantially square to its longitudinal axis, then the alignment of the two masses that are to be joined is probably the most straightforward. This is true because either rod could be rotated about its longitudinal axis and its cut face would still be properly aligned with that of the other rod. In such a joint, the rod is cut square to its length, the sections are held one against the other in some type of guide, and they are then heated and brazed. The disadvantage of this method is that the square cut results in a minimum area of rod face that is exposed to the brazing material for bonding. All other factors being equal, the joint so obtained is the weakest butt joint that is possible. Stronger joints are created when the surface area being bonded is increased. The maximum area would be exposed as the angle of the cut approaches 180 degrees. This 180 degree cut is a longitudinal splitting of the rod and can be quickly seen as impractical and worthy of no consideration. A compromise between the two extremes is the proper choice. An angle of 135 degrees, or 45 degree if the internal angle is measured, is an angle that provides an acceptable compromise of increased bonding area for a given diameter rod, while at the same time is an angle that can be reasonably worked with to produce proper gap spacing and uniform facial alignment.
Another consideration in forming a joint by brazing is the effect of thermal expansion of the parent metal as the joint is heated and the thermal contraction of the parent metal as the joint is cooled. If the two rods are each held in a fixed device to keep them aligned, the joint will be placed in compression as the parent metal expands upon being heated. Likewise, the joint will be placed in tension as the parent metal contracts upon cooling. This compression and tension can adversely affect the joint formed by brazing. Expansion of the parent metal can result in a compression that will force the gap together and expel too much filler material. Upon cooling and contracting, tension can actually tear the parent metal from the filler and weaken the metallurgical bonds in the joint being formed.